Polyurethanes are synthetic polymers of great commercial and industrial importance. Characterized by carbamate (—NH—CO—O—) chemical linkages, polyurethanes are commonly prepared by reacting a multifunctional isocyanate with a multifunctional diol or polyol in the presence of a catalyst. Thermoplastic polyurethanes are characterized by linear polymeric chains having self-ordering block structures, while thermoset polyurethanes are highly crosslinked by covalent bonds.
Depending on the diisocyanate and diol or polyol constituents used to make the polyurethane, these materials can be engineered to display a high degree of chemical resistance and a wide range of material properties. Generally, polyurethanes are extremely durable and flexible, making them desirable materials for many applications. Such applications include, for example, high-resilience foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, hoses, elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives, coatings and sealants, synthetic fibers, and carpet underlay.
Polyurethanes films and coatings can be especially advantageous in protecting substrates from environmental weathering, chemical exposure, heat, and/or abrasion. Polyurethane coatings and films can withstand harsh environments, making them suitable in outdoor applications.
International Publication No. WO 1994/013465 discloses a polyurethane-based protective layer for protecting an embossed multilayer film. This reference discloses that a surface layer containing the reaction product of an aliphatic diisocyanate and a polyester polyol, polycarbonate polyol or polyacrylic polyol can provide adequate protection in both indoor and outdoor applications. It is further disclosed that the polyurethane-based protective surface layer can be either crosslinked or uncrosslinked and can be solvent borne.
International Publication No. WO 1994/013496 discloses a multilayer graphic article having a substrate, one or more color layers disposed on the substrate, and a protective surface layer. The protective surface layer may comprise a polyurethane obtained from the reaction of an aromatic diisocyanate and a polyether polyol or a polyurethane obtained from the reaction of an aliphatic diisocyanate and a polyester polyol, polycarbonate polyol or polyacrylic polyol.
International Publication No. WO 1993/024551 discloses aqueous polyurethane dispersions including a dimer acid and low molecular weight cycloaliphatic components for coatings on various substrates. The coatings may be protective or decorative, and can provide desirable properties such as chemical resistance, water resistance, solvent resistance, toughness, abrasion resistance and durability.
We have previously publicly disclosed blackout film constructed with a polyurethane layer disposed on a black polyvinyl chloride film. The polyurethane layer was based on polyester and polyether polyols containing 6 wt % of an acrylic polydimethylsiloxane, and did not have sufficient optical properties for clear film and coating applications.
One technical problem with conventional polyurethane coatings relates to stain resistance. Even chemically-resistant polyurethanes can permanently stain when contaminants become adsorbed onto its surfaces, resulting in poor aesthetics. Such stain resistance can be enhanced by creating a low surface energy surface that induces contaminants to “bead” on the surface, thereby facilitating removal. Prior efforts to create a low energy surface focused on building polydimethylsiloxane into the polymer backbone using, for example, a dicarbinol polydimethylsiloxane or diamine polydimethylsiloxane. These compositions are disclosed in U.S. Pat. No. 5,691,439 (Slack et al.), U.S. Pat. No. 6,271,332 (Lohmann et al.), and U.S. Pat. No. 7,732,055 (Nesten et al.). Unfortunately, these approaches tended to provide porous structures with poor silicone presence at the film surface, which impaired resistance to moisture, chemicals, and staining.